Sciencemadness Discussion Board

Reaching higher temperatures

IrC - 28-8-2005 at 10:27

I have been finding that running my kiln at 2,350 F (cone 10) is hard on my money. Even though the Olympic kiln is supposed to be rated a 10 with 3 inch brick, just under two hours at 2,300 F burned the element right in two killing my operation. It appears that the metal itself of the heating coil chemically reacts with brick material forming a black ash which ends up in the metal structure failing. I have found that to make aluminate glow powders from scratch I need to reach 2,500 F to 2,600 F to get good results, beyond what electric heating wire can stand. What methods other than gas can I use to get these high temperatures sustained for hours.

Does anyone know of an affordable heating wire that can be used to rebuild my kiln that would be better than the standard factory replacement?

neutrino - 28-8-2005 at 12:16

What about an electric arc furnace?

Carbon rods can be used in an arc furnace

Lambda - 28-8-2005 at 13:26

Yes, I agree with Neutrino, an arc furnace is the way to go. I use Carbon rods connected to my arc welding machine. The ultraviolet radiation thus emitted, is extremely dangerous, and has to be shielded off at all cost. The skin burns obtained by exposure are devestating, an blindness (permanent) is a real concern when using this principle of heating. These electrodes have to be tipped together, to get the arc starting though, and gradually moved away from each other wants the arc has started. High voltage ignition may also be used, if these electrodes are at a fixed distance from each other. Due to electrode wearing, they slowly have to be moved towards each other to compensate for loss. IIRC: the first arc lights worked at 55 volts DC (22.5 volts ?), and for this reason we have 110 volts AC or 220 volts AC from the mains supply. This voltage was the result of the original arc-light DC voltage being doubled etc. Remember, in thoughs days there was a lot of debate about the mains voltage being AC or DC. I think this discussion was between Edison and Tesla at that time (many others may have been involved though). You can still find lots of information on this original arc-light design and application on the internet. The trick was, to compensate electrode loss, by slowly moving the Carbon rods towards each other, and thus obtain a stable arc and light output. Welding supply shops still sell Carbon rods (1 cm diameter and 30 cm long wrapped in with a thin Copper foil mantle) in Holland, but this may be an old supply, for they now use Tungsten and/or Tantalium alloys in TIG and MIG welding machines.

Remark:

The problem with high melting point metals and alloys, is that they often have to be used in an inert atmosphere. Tungsten wire is not only very brittle (if you want to make coils of this wire, then it becomes problematic) but also extremenly succeptible to oxidation. In practice, "inert" wire is used that is limited in respect to the maximum obtainable temperature in atmospheric conditions. Information obtained from kiln oven manufacturers may help you out, as to what wire alloy they use and to what temperature it may be used.

[Edited on 29-8-2005 by Lambda]

IrC - 28-8-2005 at 15:07

Thanks for the ideas. I know I need to keep searching for a better way, as carbon and iron are two elements which destroy flourescence and phosphorescence, and I cannot think of any way to use any form of arc which would not contaminate the materials. I was thinking of finding a quartz tube a few inches in diameter and building a tube firnace around it. This way I could try some of these ideas and still keep my materials chemically pure. Actually this would also allow an easy way for me to run 3%H2+97%N2 gas through while hot to reduce the europium from a 3 to a 2.

The problem I have is these commercial kilns were designed to run cone 10 (2,350 F), but the kiln sitter shuts it off at 10 meaning the firing run is done. In my case I get it up to 2,300 F or so and keep it there for hours at a time, and nobody designs kiln to run like this in the affordable marketplace of ceramic making. I wish I could find a killer deal on a scientific kiln which was designed for the things I am doing. I also use it to reduce my carbonates to oxides, and I am sure part of my problems stem from chemical vapors eating away at the heating elements.

More and more it looks like I am going to have to design a tube furnace around a piece of quartz tubing, but this also is real hard to find in a 3 inch diameter unless you are buying an 8 foot length for 500 dollars. This homebrew science thing is for the wealthy.

Lambda - 28-8-2005 at 16:01

IrC, you have made it clear that your ultimate goal is to obtain a pure sample. But, you don't have to go all the way in this. You can protect your sample with a protective powder layer, which will introduce impurities though. This does not necessarily mean that the whole sample will be contaminated. You are conducting research work, and is it not your ultimate goal to abtain even a small sample which may be uncontaminated, if only to prove the feasibility of a certain mix ?. The limitations in which you may have to work will ultimatly imply more loss of product, however, you may still obtain your objectives in this way.

[Edited on 29-8-2005 by Lambda]

neutrino - 28-8-2005 at 18:06

What exactly do you need in a furnace? Movement? Volume? Why would you need a 3" tube?

Fleaker - 28-8-2005 at 18:11

Could you perhaps construct your own kiln? It's not too difficult and you can buy elements good to 1800C (molybdenum disilicide) yet those are expensive, still they last long.

Alternatively, if you use an inert atmosphere, you could use SiC, C, or even W elements to get high temperatures and they last well with chemical vapors and have good service life. If you only need 1400C, just look for a good quality element from Kanthal, I'm sure they have some that will meet your requirements.

Eitherway, if your kiln is only good to cone 10, then you would need to put some better refractory (1700 castable, great stuff, very durable) and some IR reflecting compound (i.e. ITC-100, zircon based). If you do reline the kiln with a high alumina refractory, there will be less fluxes to ruin your elements. I suggest you try an application of ITC-100 on the walls: higher temperature, and it won't react with your elements.

My experience with quartz tube furnaces (TGAs) is that you can get the temperature up rather quickly, as much as 60C per minute but as you mentioned, it's excessively expensive for quartz anything.

As to what Lambda said about tungsten being brittle, my experience with it is that at a dull red heat, it's rather ductile and can be extruded through dies with relative ease (when making wire).

Now electric arc furnaces (reduced pressure inert gas) are hard to control with respect to temperature and I find that you get a lot of volatization of metals like titanium. However, the more refractory metals such as tungsten, tantalum, and rhenium take longer to melt but are more prone to carbide formation.

12AX7 - 28-8-2005 at 18:35

Pffbt, you don't want arc. Gas or best, electric, is the way to go here. They make elements for reducing atmosphere at that temperature. Molybdenum and tungsten would last just fine if you make sure there's always hydrogen. (BTW does it HAVE to be hydrogen, or can you use fuel gas (natural gas (methane), propane, etc.) instead?) There ought to be cheaper nichrome or such alloys available for that temperature range.

I would recommend an inch or two of 2600 to 3000°F (uh 1400-1600C) castable, plastic moldable or soft brick type material. Cut or mold notches into the face for the elements. I'm guessing you don't need cubic feet here for your samples, so that should be easy and cheap enough. Then back that up with a layer of kaowool.

Tim

IrC - 28-8-2005 at 19:25

Great ideas people! I am going to do some searching and find the materials to build one that can do close to 1,800 C. Quartz is the only thing I can think of to contain the samples. What I need is a tube that runs through the kiln so that I can run hydrogen gas through it, without ignition at these temperatures to reduce the europium. I had decided on electric as gas would contaminate the samples as hydrocarbons do contain carbon. Now if I could find an affordable way to get tanks of pure hydrogen I could run the mixture slightly rich and accomplish the reduction with the very source of heat. That would be really cool. Of course I have no clue where around me I could refill H2 tanks, let alone even get them. So being realistic of course I have to go the electric route. I did not know about some of the special materials mentioned here, I will search for them but if anyone knows where to buy them in hobby quantities please post the information here.

What you do is mix compounds like Al2O3, Sr(CO3)2 (which I first ran through kiln at 1,100 C for an hour to get rid of the carbon), La2(CO3)3 (same thing, reduce to La2O3 first), Eu2O3 (I put europium metal in beaker with paper towel cover for a few days to allow it to turn into Eu2O3), and various co-activators such as Dy2O3, etc., ball mill the hell out of them, and run them through the kiln. I also play with calcium aluminates as well as strontium aluminates, making really cool glow powders. My batch that murdered my beautiful stainless steel kiln was an experiment using titanium and silver metals as charge compensators, as I was trying to experiment with glow powders which only respond to radiation, in trying to come up with novel new materials to cover scintillation screens. But alas this is bottled back up and on hold as now I am in search of parts to rebuild my kiln.

As to the carbon contamination thing, this I learned in my quick and dirty method. I chanced upon a hundred alumina boats and made a setup to hold them so I can fill them with trial runs and blast the hell out of them with an oxy/mapp torch. I have found that with a given batch even a slight movement of the flame into the unburned fuel portion gives a result that would not glow at ground zero in an old "B rated" nuke movie. But carefully keeping the flame on the material in such a way as no unburned fuel ever touches the boat, I get small amounts of glowing compositions. In this way I can in 10 minutes figure out if a mixture is going to work before I do a whole batch in the kiln. I do work on a budget for these experiments, and the mapp thing sucks as the oxygen tanks run out in 3 minutes and cost 8 bucks a shot. But my oxyacetylene outfit was robbed from me so I have to go with what I have until I can replace it.

Something that would be a lot easier to do if I had not spent $30,000 on various elements and chemicals in the past year. Oh shit, I think I am whining again.

darkflame89 - 28-8-2005 at 23:51

Wat about the combustion between hydrogen and oxygen? A oxyhydrogen blow torch. Well, you can achieve high temps with that. The problem is to make the setup safe. Another disadvantage is that the flame will be small and intense, because you will need a lot of hydrogen. You can construct an electrolysis cell and does this specially. Use a adapter and use electricity from the wall or batteries. Decrease voltage to bare minimum of 1.5 V and increase current as much as possible, thus maximising power. The resulting hydrogen can then be burnt:cool:

IrC - 29-8-2005 at 00:12

I actually thought about trying such a setup but it is dangerous and hard to perfect. The comment about lowering voltage and raising current will not work as it is volts that push amps. This means for a given conductivity the only way to increase current is to raise the voltage, and the resistance in an electrolytic cell will be considerable compared a metal such as copper. Therefore it would mean raising to an even higher voltage to increase the current flow as compared to the current in a wire. Hopefully I explained that well enough, the fact that it is impossible at a given cell impedance to hold the voltage constant while raising the current. Of course this is not taking into account any effects of a negative resistance which is possible in such a cell, but it is likely in a real world hydrogen generation cell the voltage will have to be raised to increase the current.

The oxyhydrogen flame would be so perfect for what I am doing that it sucks that such equipment is not readily available, like I stated earlier I doubt I could refill a hydrogen tank within several hundred miles of where I am anyway.

Fleaker, I forgot to mention but your information is so much in line with improvements I need to make. It appears you have much experience with this subject and I hope you have sources of these items you mentioned which you can post, as so far you have convinced me to completely rebuild my kiln with better elements and better refractory lining.

[Edited on 29-8-2005 by IrC]

neutrino - 29-8-2005 at 04:53

There are devices on the market (torches, I think) that create oxy-hydrogen flames simply by the electricity from a wall socket. Perhaps you should look into one of those?

For quartz, take a look at these two companies:

http://www.qsiquartz.com/prod.htm
http://www.technicalglass.com/prods.htm

darkflame89 - 29-8-2005 at 07:27

Yes, the volts push amps matters if u are using batteries. Now, if you are using electricity from the wall, basically AC current, it is possible to to get minimum voltage and maximum current. The electricity going into ur house is high voltage and quite high current(depends on your point of view). If you use a adapter to step down the voltage to "usable" levels, you can draw more current. Because to the generator what matters in the amount of power drawn.

And, you can connect a few of those 1.5V D cells in parallels to give off a larger current. And, you can add a few of these electrolysis cells together in series, produce more hydrogen for a given current. Something about large current density for the electrodes comes in here. I'm not too sure about the stuff about current density, perhaps other more experienced people care to explain this part?

For your information, there is this link where the person has experimented with home-made oxyhydrogen torches:
http://www.oupower.com/index.php?dir=_My_Projects/_Over_Unit...
Yeah, I know this guy is obsesses with over unity power, just ignore the parts about the over unity thing..

IrC - 29-8-2005 at 08:12

"Yes, the volts push amps matters if u are using batteries"

I=E/R, no matter where you get the electricity. When it comes to AC power you are still dealing with ohms law, you are just dealing with a cycling voltage where terms such as peak, average, and root mean square come into play.

"current density"

Just a measure of current flow per unit area, it is current density that is responsible when you overload a circuit and burn a wire open.

Thanks for the links, neutrino and darkflame89, I will check them out.

[Edited on 29-8-2005 by IrC]

Oxydro - 29-8-2005 at 21:53

There is a thread "Construction of oxyhydrogen blowpipe" at http://www.sciencemadness.org/talk/viewthread.php?tid=3203

As I think I might have written there, I tried it but quickly gave up when I found it was much more fun to shred things by igniting bottles of the gas.

unionised - 30-8-2005 at 02:57

IrC
When did electrolytic cells, like those used for hydrogen generation, start obeying Ohm's law?
For H2 generation it's more efficient to use a low voltage (as low as you can reasonably get) at a high current.

Also, the current density, among other things, affects the cell overvoltage. In this instance, you want to keep it low. For some electrochemical syntheses you want it high.

Eclectic - 30-8-2005 at 05:29

The Kanthal and nichrome type heating elements are quickly destroyed by cycling between oxidizing and reducing atmospheres at high temp. The surface oxides that protect the rest of the element spall off. Thicker wire for the elements will give longer operating life. Try to isolate the reactants and reducing atmosphere with a muffle chamber, maybe some sort of large crucible of high alumina ceramic.

IrC - 30-8-2005 at 12:30

"When did electrolytic cells, like those used for hydrogen generation, start obeying Ohm's law?"

Electrons obey ohms law, no matter where they go, and sometimes they go places where the math gets complex. However, this does not change a thing, unless you put your cell inside a black hole it will still always obey the laws of the universe.

It is not magic, just science!

Ohm, Amps and Voltage only exist together

Lambda - 30-8-2005 at 16:03

IrC, if I am not mistaken, Unionised means that high voltages cause erratic hugh gas bubbles. These big gas bubbles (and steam) then start to isolate the electrode surface, bringing the resistance up, and the relative active surface aria down. Lower voltages cause more regular small bubble (less steam) to evolve. The current and voltage product may in this case be more efficient (less steam), in respect to the power input and the amount of gas produced. Ohms law is of course appliable (in both cases), but in a less erratic form.

IrC - 30-8-2005 at 18:12

I understand these things, but I was just replying to the comment about raising the amps while keeping the voltage constant, not to be confused with criteria for what are the correct voltages in a given cell. In effect the only way to raise the current at a constant voltage is to lower the impedance of a cell (or any other resistive element in any circuit). This could be done by increasing the surface area of electrodes, moving the electrodes closer together, or by increasing the conductivity of the cell. Of course you are likely also dealing with aspects of the cell which cause impedance to not be a constant property, probably meaning some type of regulation circuitry would be a good thing.

I forgot to mention but my comments were in reference to the statements applying to battery versus house power, where I was stating that this consideration is wrong, (of course assuming the AC was rectified). I am sure AC in the cell would cause other problems involving ion mobilities and debye distances and so on.

[Edited on 31-8-2005 by IrC]

darkflame89 - 30-8-2005 at 19:47

Watever, you just need to connect a few 1.5V D batteries in parallel and connect, say, 2 electrolytic cells in series.(The batteries goes flat like in less than one day) Increase the electrode surface area by connecting a few of those steel plates together and you got yourself a working cell. The worrisome part is how to pressurise the gas enough so that gas can be vented safely and burnt. If there's not enough pressure, the flame might be sucked back into the tube and straight into your electrolytic cells, and BOOM! Everything within 1m radius will be obliterated instantly. Oh yeah, you could pipe the gas into a gas syringe; the gas vents from the needle(metal, not glass) and you can burn it. But the flame is small and extremely intense, as well as being colorless.

[Edited on 31-8-2005 by darkflame89]

IrC - 30-8-2005 at 19:54

You just reminded me of what I was going to ask you earlier. Why could the cell not separate the O2 and H2 and then they could be recombined in the burner chamber like a rocket motor (sort of), to eliminate all dangers of having an O2/H2 mixture hanging around?

[Edited on 31-8-2005 by IrC]

neutrino - 30-8-2005 at 20:55

That should work. See my previous post.

darkflame89 - 31-8-2005 at 02:21

Combine them in a burner chamber, but that would not be controlled, it would go with a bang and that's it, isn't it? Unless I got you wrong, else this would only give bursts of heat, not to say damaging to your burner.:o Burning it the traditional way like a Bunsen Burner would give controlled burning.

neutrino - 31-8-2005 at 03:45

The same thing is done sucessfully with H<sub>2</sub> + Cl<sub>2</sub>. (industrial HCl production) :o

I don't see why this couldn't work.

unionised - 31-8-2005 at 11:55

Electrolysis is used for hydrogen production, certainly on a lab scale.

"Electrons obey ohms law, no matter where they go, and sometimes they go places where the math gets complex. However, this does not change a thing, unless you put your cell inside a black hole it will still always obey the laws of the universe. "

Bollocks!

Plenty of things have non linear I vs V curves, electrolytic cells are among them. Semiconductors and gas lamps are some of the better known ones.

Google finds about 13500 hits for "non ohmic". Are you going to write to them all and tell them they are wrong?

[Edited on 31-8-2005 by unionised]

IrC - 31-8-2005 at 14:34

How can you be so good at chemistry and say something like that? Electrons obey all the physical laws of the universe whether or not you believe it. Just because you are dealing with complex ( and by this I include non linear) and changing impedances does not change a thing. Electronics is a subject I am well versed in having run a hundred million dollar R&D lab where my function was engineering and reverse engineering various technologies. I find it simply astounding to hear such opinions as if electrons follow some magical route just because chemistry is involved. Go back to the statement I first responded to and read it again. The statement was that somehow batteries made a difference in current versus voltage as compared to a line derived power source. I hope you are planning on taking at least some physical science along with your chemistry in order to have a better idea of what you mean when you say "Bollocks!".

It does not matter whether or not the I versus E curves are nonlinear or not, there is still a mathmatical formula to predict the values of each parameter, and you will find that the laws of science are still being obeyed even if you do not like it!

Let me explain this in simple terms for you. Ohms is opposition to the flow of electrons intrinsically. So in any circuit no matter what the conditions a given voltage will produce a given current with a given impedance to the flow of said current. I=E/R still prevails, you must simply find out what the complex value of R is, and R may be negative but the math still works. Maybe I should write them and tell them they are all wrong since the term "non ohmic" is in itself a misnomer. A bad term. Simply because the opposition to current does not follow a linear curve does not mean there is no opposition to current flow, which again is the definition of the term ohms. Why would you wish to make an argument out of my previous statements? Merely because you did not understand what I was saying? There must be better things you can do with your time.

Looking at a source of power as a generator with a voltage, current potential, and internal resistance, it makes no difference where the power comes from, house current or batteries, conditions outside of the generator in the same circuit will not change, and this was the point of my statements. You are creating a whole new subject to argue about, for some reason I do not see. I mean think about it. You quoted my statement that electrons obey the laws of physics and you come back to this statement with "Bollocks!"! Do me a favor. Print this all out and take it to your physics professor, and write back here what he or she has to say about it.

[Edited on 31-8-2005 by IrC]

[Edited on 1-9-2005 by IrC]

12AX7 - 31-8-2005 at 16:51

...Non-ohmic refers not so much to the instantaneous V/I = R points on the V vs. I graph but more to the derivative, which is in units of rise/run = volts/amps = ohms (dynamic impedance in this case)...

Tim

IrC - 31-8-2005 at 17:45

That much is clear, but I for one would be happier if they would change the name from "non-ohmic" to something more correct in description. That's all I'm complaining about. Who coined that term anyway and who do we call to get it changed to something more appropriate? To me "non-ohmic" describes a superconductor, or something like one.

Or in general "ohmic" means anything which obstructs the movement or lowers the kinetic energy of an electron in motion. If a field is applied in the circuit there is an energy loss which results in the need to apply more energy to maintain the uniform motion of charges. So we see that to increase the velocity of the charges and compensate for the kinetic energy lost in collisions, we have to increase the accelerating voltage or field. This is why volts push amps.

[Edited on 1-9-2005 by IrC]

unionised - 1-9-2005 at 11:31

Are we at crossed purposes or what?
This is what I mean by Ohm's law

http://micro.magnet.fsu.edu/electromag/java/ohmslaw/

it's the one Ohm postulated a long time ago. The part that I'm talking about is the bit where it says proportional. That's what Ohm said- that's his law.
Any V/I relation that isn't linear (ie where I is not directly proportional to V) is a deviation from Ohm's law.

You can define a dynamic resistance (or impedance) if you want but the law that the bloke wrote is very simple and electrolytes don't generally obey it.

Another sense in which you are clearly talking through your hat is that you keep on talking about electrons, it may have escaped your notice that the current carriers in solutions are not electrons so your whole argument is based on an irrelevance.
That's not the whole story. The V vs I relation is about current- the rate at which electrons pass through a circuit. |If you only have one electron it either does or does not pass a point. If it does then the current is infinite (finite charge, zero time). If not the current is zero.
Given that current is that poorly defined for a single electron, saying it obeys Ohms law is meaningless.

Before you tell me to study some physics perhaps you should take the time to learn what you are talking about.

"Electronics is a subject I am well versed in having run a hundred million dollar R&D lab where my function was engineering and reverse engineering various technologies."
And the fact that you did that without taking the trouble to learn Ohm's law makes it so much more impressive in one sense, but like empty bragging in another.

BTW, one of my drinking buddies is an electronics lecturer- would you still like me to ask his opinion of your work? I'm sure he would enjoy it.
And to answer your first question about how I can be so good at chemistry- well, I studied it for a while then used it to earn a living for a couple of decades. At least most of the time I was doing chemistry- sometimes, when the section needed some electronics doing, they asked me.

[Edited on 1-9-2005 by unionised]

Twospoons - 1-9-2005 at 15:12

Quote:
Originally posted by unionised
(finite charge, zero time).


Uh, sorry but current is generally in units of coulombs per second, so the 'time' is NOT zero. So for a single electron the current would be either unbelievably small (1.6 e-19 amps) or zero if it doesn't pass in that second.

Oh, and I agree that 'Ohm's Law' refers to linear resistance. And in real life non-linear V/I relationships are very common.

IrC - 1-9-2005 at 16:00

unionised, this all started as I was trying to get someone to see that a line powered supply can do the same job as a battery supply, and you are turning my kiln thread into a bunch of useless argument about how you can have current flow with no electrons at all moving in the circuit. Why bother even hooking wires to your cell? I guess then only protons, holes, and bacteria are moving in your circuits but I for one could not care less, the point of this thread was my kiln. I don't need your insults to my knowledge, intelligence, abilities or past work I have done. Should I post here what I think you and your drinking buddy can go do with your opinions of me?

I think you should go start your own thread and talk about how wonderful you are there, and leave my kiln subject alone. I have better things to do with my time.


[Edited on 2-9-2005 by IrC]

unionised - 1-9-2005 at 21:56

Twospoons, if the charge takes half a second, the current is twice as large, if it takes .1 sec the current is 10 times as large
Since the time taken for an electron to pass a mathematical point (ie with no size) is zero, the current is infinite.

IrC,
So, when do you admit that you totally failed to understand Ohm's law then? Is that when you apologise to Darkfire for not understanding the impedance matching idea? When do you explain to me how you were so conceited that, rather than check on the accuracy of those 13500 folks, you decided you were right and patronised me?

jpsmith123 - 2-9-2005 at 02:43

Hello unionised,

I don't think that's quite correct; since an electron has finite size, finite charge, and finite velocity, the current resulting from its passage cannot be infinite.

Regards,
Joe
Quote:
Originally posted by unionised
Twospoons, if the charge takes half a second, the current is twice as large, if it takes .1 sec the current is 10 times as large
Since the time taken for an electron to pass a mathematical point (ie with no size) is zero, the current is infinite.

darkflame89 - 2-9-2005 at 06:49

Well, all these arguments aren't really that neccesary, what I had meant all along was that to power the cell, its more efficient to have very low voltages and increase current as much as possible. Yes, agreed that when you pump electrons(amps), you need energy(volts). But current is limited by resistance and voltage. What you mean when you say that by to increase current, you need more voltage. That is true, when you have fixed resistance. Then, to pump more amps, you need more volts.

What I mean is that you do not neccesarily need more volts to pump more amps. Because ultimately, batteries and AC power supply are determined more accurately by power. P=VI. That is, for the same power, you can have more volts but less amps. Conversely, you can more amps, but less volts. For the batteries, this means that if you draw a larger current by lowering resistance, you draw more power and the batteries goes flat rapidly; not at all what we want for we want a sustained production of hydrogen gas, we certainly do not want the battery to go flat in 10 mins.

Instead, we can use the wall adapter, the wall adapter steps down voltage, but steps up current. The power station provides a certain amount of power. Its up to you to use allocate the power to push only a few electrons with a lot of energy, or push a lot of electrons, but all them having lower energy. That is how you can lower the voltage as much as possible, but still increase current.

What unionised means is not that Ohm's Law is not obeyed. There's a relationship between resistance, ohms and volts, yes, no one's denying that. But Ohm's states that the relationship is linear, that is, the graph of the current vs volts is a line. But from normal experience, we know that it is not neccesarily a line. So this sort of conductors is termed non-ohmic. Which means that they do not conform to the linear rule. Non-ohmic does not mean that the conductors do not obey the part of volt push amps.

12AX7 - 2-9-2005 at 07:38

Quote:
Originally posted by unionised
Since the time taken for an electron to pass a mathematical point (ie with no size) is zero, the current is infinite.


What the? Oh, I see, you slept the week when the prof was going over current flow. Something about a variable, vector even, named J.

Current is electrons <B>passing through a cross section.</B> At t=0, they were on one side, at t=1 they passed it. No infinities.

Tim

IrC - 2-9-2005 at 11:54

You need merely think of a generator and compare the internal impedance of one to another. More current capacity does not dictate that more amps will flow, it depends upon the circuit. I didn't bother to get into mobilities, and the fact that the electrons in a cell have the highest mobility. In fact, I give up on this entire line.

Can we go back to talking about kilns and improvements in same?

Fleaker - 2-9-2005 at 18:47

Try this IrC:

if you plan on making high temp stuff, this is ok (wouldn't buy the refractory blanket or castable, but the brick is good)

http://www.uu77.com/groupslist.aspx?CategoryID=109&Categ...

unionised - 3-9-2005 at 02:33

I will just deal with the current current debate here.
It takes just the same time to pass through a cross section as it does to go past a point.
"Current is electrons passing through a cross section. At t=0, they were on one side, at t=1 they passed it. No infinities.
"
In what units are you measuring T? t=1 day, 1 hour, 1 nanosecond?

The point I was making is that at t=0 the electron is on one side; at T=delta t it is on the other side. As delta t tends to zero the rate of change of charge dQ/dt tends to infinity.
The instant the electron clears the starting gate the charge goes from nil to about 10^-16C. That happens in zero time, at one instant it's on one side, the next instant, its on the other. The time interval is vanishingly small so the current dQ/dt is infinite.
The average current over any practical time scale is small, but the instantaneuos cuurent is infinite*. There's a matematical function to describe this called something like "Kroneker's Delta" an infinitely narrow, infinitely high spike with unit area.

Anyway, the fact that we are having this debate proves my point.
Ohms law talks about currents and currents, as we have seen, are poorly defined for single electrons so IrC's assertione that every (single) electron obeys Ohm's law is clearly not true.
Any further discussion about current with single electrons will just reinforce this point.

IIRC Ohm was dead roughly 50 years before the electron was characterised- He was a clever man, but not that clever, he didn't include electrons in his law.

I will set up another thread to avoid cluttering this one further while setting the record straight.
Here it is.
http://www.sciencemadness.org/talk/viewthread.php?tid=4405&a...


* actually, since attosecond timings are possible and picoseconds are quite commonplace, this is not strictly true one electron per picosecond is a perfectly respectable current. Since the debate was about single electrons it's not an important matter


[Edited on 3-9-2005 by unionised]

jpsmith123 - 3-9-2005 at 05:16

The time it takes an electron to traverse a geometrical point in space is simply the electron's "length" (diameter) divided by its velocity, no? Since both of these quantities are finite, the current is also finite.

Using the "classical" electron radius of about 2.82 x 10^-15 meters, a velocity of 0.1 c (ignoring relativistic length contraction) and a charge of 1.6 x 10^-19 coulomb (your value of 10^-16 is incorrect), I get a very finite current of about 851 amps.
Quote:
The point I was making is that at t=0 the electron is on one side; at T=delta t it is on the other side. As delta t tends to zero the rate of change of charge dQ/dt tends to infinity.
The instant the electron clears the starting gate the charge goes from nil to about 10^-16C. That happens in zero time, at one instant it's on one side, the next instant, its on the other. The time interval is vanishingly small so the current dQ/dt is infinite.
The average current over any practical time scale is small, but the instantaneuos cuurent is infinite*. There's a matematical function to describe this called something like "Kroneker's Delta" an infinitely narrow, infinitely high spike with unit area.
...[Edited on 3-9-2005 by unionised]

12AX7 - 3-9-2005 at 08:20

Quote:
Originally posted by unionised
"Current is electrons passing through a cross section. At t=0, they were on one side, at t=1 they passed it. No infinities.
"
In what units are you measuring T? t=1 day, 1 hour, 1 nanosecond?


...WTF...?! How fucking desperate ARE you?

<B>You know as well as I do that time doesn't matter, it's all factored when going back to coulombs <I>per second</I>!</B>

Going to dt --> 0 doesn't work for the express reason that nothing happens in zero time. I guess you forgot that before you made an ass of yourself... (where's that rolleyes smiley)

And besides, quantum theory applies at this scale (obviously). Ohm's law works quite nicely for averages of many electrons. Ohm's law still works, but you keep trying to undermine it with dubious statements designed not to argue scientific fact but to publicly masturbate your ego. Which reminds me, since this post is getting somewhat heated and you will certainly reply to it, in an attempt to draw me into the fray, this will probably be my last word on the subject.
Quote:
Using the "classical" electron radius of about 2.82 x 10^-15 meters, a velocity of 0.1 c (ignoring relativistic length contraction) and a charge of 1.6 x 10^-19 coulomb (your value of 10^-16 is incorrect), I get a very finite current of about 851 amps.


There you go, a finite answer using your imagined situation <U>properly</U>. You were saying?

Tim

unionised - 3-9-2005 at 08:34

I'm not desperate yet, and I don't think I will be any time soon.
The point I was making is that one second is a totally arbitrary time and that the interval, delta t, that you need to divide the change in charge (deltaq) by to get the current isn't 1
It isn't 1 second, it isn't 1 day. It's the time taken for an electron to cross the line.
If the electron were a block of wood with the size and velocity you give it then the time interval would be as you calculated and the current would be the value you gave it.
The problem is that we are talking about electrons.

The size of an electron is poorly defined but the value you have use is perfectly reasonable, the trouble is that it's classical.
You seem to have forgotten that while you made your "propper" finite calculation. Who exactly has made a fool of themself?
However, I take the view that, since the elecrons wave function is infinite in extent you cannot use it's size in this way because, if you do, then it takes an infinite time for an electron to cross from one side to the other- an equally absurd notion.

The best way forward that I can see for expressing the position of an electron is to use the maximum of its wave function.

That, of course, is a mathematical point and takes no time to cross the line.

You can take any value you chose as the size of an eletron in this context. A mathematical point or an infinite wave function. You seem to have chosen a value in between those two extremes and calculated from it as if there is some God -given reason to say it's correct.
With an ego like that I'm sure that you can masturbate it wherever you want.

All of this goes to prove my point that, since Ohm's law refers to current and current isn't well defined for a single electron, IrC's assertion that Ohm's law holds for every electron is questionable at best.



BTW, Perhaps it would be better to move this discussion to the other thread I set up.



[Edited on 3-9-2005 by unionised]

[Edited on 3-9-2005 by unionised]